Upon activation by antigen, B cells undergo antibody class (isotype) switching, changing from expression of IgM to expression of IgG, IgA or IgE, while maintaining specificity for the same antigen. Since the isotype determines the effector function of the antibody, class switching allows the humoral immune response to adaptively respond to different infectious organisms. Class switching occurs by a DNA recombination event between switch (S) region sequences located upstream of each heavy chain constant (Ch) region gene. Numerous studies (including many from this laboratory) have established that transcription of specific unrearranged Ch genes occurs prior to switching and that this transcription is required for switching. Recent data, however, indicate that isotype specificity of switch recombination is regulated at additional undefined levels. Regulation of IgA switching is especially mysterious because it can only be induced to low levels in culture and yet is the most abundant antibody class in the body, expressed specifically in mucosal tissues. IgA is an important first line of defense against pathogens entering through body orifices. Data from our lab indicate that LSF/CP2/LBP-1c binds the switch (S) region DNA sequences associated with the IgA Ch gene and regulates switching to IgA in the l.29u B cell line. We have also found that the histone methyltransferase Suv39h1 specifically increases IgA switching in a plasmid switch substrate. The goal of this proposal is to investigate the mechanism of this regulation and to provide evidence for the hypothesis that chromatin accessibility and histone modifications of specific S regions regulate class switch recombination. In Aim 1 we propose to investigate the role of Suv39h1 in endogenous switch recombination and the mechanism of its function. We will investigate the hypothesis that Suv39h1 inhibits the activity of a sequence-specific DNA binding protein to repress switch recombination to IgA. The most likely candidate for this inhibitory protein is LSF/CP-2/LBP-1c, which is known to bind histone deacetylases and polychrome group proteins. Aim 2 will be to determine whether LSF specifically inhibits IgA switch recombination in normal splenic B cells and to examine the mechanism of inhibition of IgA switching by LSF.